Generally, copper-nickel alloys exhibit excellent natures of corrosion resistance, ductility, processability, and high temperature properties by changing the ratio of copper and nickel, and also have characteristic natures such as electrical resistivity, heat resistivity coefficient, thermal electromotive force, and thermal expansion coefficient. Thus, studies have been conducted from the past to obtain such properties of copper-nickel alloys by electroplating. Numerous baths have been studied such as a cyan bath, a citric acid bath, an acetic acid bath, a tartaric acid bath, a thiosulfuric acid bath, an ammonia bath, and a pyrophosphoric acid bath, as the copper-nickel alloy plating bath that has been conventionally attempted; however, none has been put into practical use. The reasons that no copper-nickel alloy plating has been practically used include: (i) copper and nickel differ from each other in deposition potential by approximately 0.6 V, so that copper preferentially deposits; (ii) such a plating bath is unstable, so that an insoluble compound such as metal hydroxide is formed; and so forth. Examples of copper-nickel alloy plating baths reported so far include the followings.
(1) JP-A Sho 49-90234:
An electroplating bath containing copper, nickel, and boric acid at a pH of approximately 1, with which a plating having a copper content of 25% is obtained.
(2) JP-A Sho 52-24133
A bath containing a mixture of copper, nickel, citric acid, and ammonia water, with which a plating having any alloy composition is obtained.
(3) JP-A Sho 58-133391:
A pyrophosphoric acid-based bath containing pyrophosphosphate whose concentration is defined, to which primary and secondary additives are added to thereby obtain a bright plating.
(4) JP-A Hei 2-285091:
A bath containing nickel sulfate, nickel chloride, copper sulfate, sodium citrate, boric acid, and further sodium borate added thereto, and having a pH of 4 to 7.
(5) JP-A Hei 5-98488:
A weak acid bath containing copper, nickel, sodium tetraborate, saccharin, and a carboxylic acid such as malic acid, gluconic acid, and salicylic acid, with which a plating having a copper content of 20 to 60% and a cupronickel color is obtained.
(6) JP-A Hei 6-173075:
A weak acid bath containing copper, nickel, an aminocarboxylic acid, and sodium heptonate, with which a plating having a copper content of 18 to 64% and a cupronickel color is obtained.